Fiber-reinforced composite materials made of a reinforcing fiber and a matrix resin allows material design taking advantages of the reinforcing fiber and the matrix resin and, consequently, their use is expanding to the aerospace field, sports field, general industrial field, and the like.
As a reinforcing fiber, glass fibers, aramid fibers, carbon fibers, boron fibers, and the like are used. As a matrix resin, both thermosetting resins and thermoplastic resins are used, but thermosetting resins, which readily impregnate into a reinforcing fiber, are often used. As a thermosetting resin, epoxy resins, unsaturated polyester resins, vinyl ester resins, phenol resins, bismaleimide resins, cyanate resins, and the like are used.
To the production of a fiber-reinforced composite material, methods such as the prepreg method, hand lay-up method, filament winding method, pultrusion method, RTM (Resin Transfer Molding) method, and the like are applied.
In recent years, under circumstances where environmental regulation of automobiles has been tightened worldwide, automobile manufacturers both within the country and abroad have been making efforts toward weight saving of automobile bodies which influences fuel consumption performance, and use of carbon fiber composite materials whose weight is about half of iron and 70% of aluminum has been actively considered. Various parts for automobiles require high rigidity and strength properties as well as weight saving, and often have a three-dimensional complicated shape. Accordingly, the RTM method, which uses carbon fibers with high rigidity and high strength in the form of continuous fibers and is applicable to the complicated shape, is an effective molding method. The RTM method is a method in which a reinforcing fiber substrate is placed in a mold and then the mold is closed; a resin is injected through a resin inlet and impregnated into a reinforcing fiber and then the resin is cured; and the mold is opened and a molded article is taken out to thereby obtain a fiber-reinforced composite material. A key issue that arises in widespread use of carbon fiber composite materials in automobiles is productivity and, because of this obstacle, their use is limited in only some luxury automobiles. In addition, when using carbon fiber composite materials for automobile parts, the heat resistance in a painting process in which the temperature generally reaches 170° C. or higher must be considered.
To achieve such a high-level productivity and heat resistance using the RTM method, it is specifically required not only that the curing time of a resin be short but also that the following four conditions be simultaneously satisfied. First, in preparation operation of mixing resin materials, each material is a low-viscosity liquid and is excellent in mixing operability. Second, in the step of injecting a resin into a reinforcing fiber substrate, a resin composition is low-viscosity, and during the injection step, the increase in viscosity is reduced and the resin composition exhibits excellent impregnating ability. Third, sufficient high-speed curing can be achieved in a low-temperature range around 100° C., thereby allowing simplification of molding equipment and eliminating the need of heat resistance of subsidiary materials and the like, leading to cost reduction, and, at the same time, thermal contraction deriving from a temperature difference between a curing temperature and normal temperature can be reduced, whereby a molded article has excellent surface roughness. Fourth, in the mold release step after molding, the resin attains sufficient rigidity due to curing and can be released from the mold smoothly without causing strain; further, strain or deformation will not occur even after a painting process, and a molded article can be provided with high dimension accuracy.
To solve these problems, an epoxy resin composition that has sufficient impregnating ability because of its low viscosity and a small viscosity increase after mixing and exhibits high heat resistance after curing by using as a base resin a combination of bisphenol F epoxy resin, phenol novolac epoxy resin, and/or epoxy resin having at least three glycidyl groups in its molecule has been disclosed (JP 59-155422 A).
Also, an epoxy resin composition that has high flowability by using as a base resin a combination of alicyclic epoxy resin and cresol novolac epoxy resin has been disclosed (JP 2004-204082 A).
Further, an epoxy resin composition that has an excellent balance between a low viscosity holding time and a curing time under constant temperature conditions around 100° C. by using an epoxy resin composition combined with acid anhydride as a curing agent and an organophosphorus compound as a catalyst has been disclosed (WO 2007/125759 A1).
Furthermore, an epoxy resin composition that has a low viscosity and exhibits high heat resistance when formed into a cured resin product by using as a base resin a combination of base alicyclic epoxy and a small amount of phenol novolac epoxy has been disclosed (WO 2009/089145 A1).
The epoxy resin composition disclosed in JP '422 was an epoxy resin composition for filament winding molding, and simultaneously achieved viscosity stability in a resin bath at about 40° C. and heat resistance after curing at 150° C. However, the epoxy resin composition does not have high-speed curability required for molding by the high-cycle RTM method, and for heat resistance, its glass-transition temperature (hereinafter referred to as Tg) was about 150° C., which was insufficient to withstand a painting process of automobile parts.
The epoxy resin composition disclosed in JP '082 is a liquid resin composition that is solid or high-viscosity at normal temperature contemplated for use in semiconductor sealing materials, and the impregnating ability into a reinforcing fiber is not considered. Further, it takes 30 minutes or more to be cured, and the epoxy resin composition does not have high-speed curability.
The epoxy resin composition disclosed in WO '759 has a problem in that the heat resistance after curing was insufficient.
The epoxy resin composition disclosed in WO '145, which was not contemplated to be molded by the high-cycle RTM, exhibited a large viscosity increase during an injection process and has insufficient high-speed curability in a low-temperature range.
As described above, hitherto there has been no epoxy resin composition that is applicable to the high-cycle RTM method and exhibits sufficient heat resistance when cured.
It would therefore be helpful to provide an epoxy resin composition that is excellent in operability in preparation of a resin, is excellent in impregnating ability because a low viscosity is maintained during injection into reinforcing fibers, and cures in a short time at the time of molding, thereby providing a fiber-reinforced composite material that exhibits high heat resistance when cured and has a high surface grade and dimension accuracy even after a painting process, and a fiber-reinforced composite material using the same.